We have 4 indoor cats, the 2 youngest are a brother - sister pair, and they grew up with a pair of trees.  I bought this to replace those trees.  They love this one just as much.  One is usually inside while the other is on a ledge.

My concern is with the two top ledges which are attached only to the top of a pole.  Over time they will wobble and eventually break under the weight of full grown cats.  This will reduce the hight but they will still be able to look out a window.